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.\" Copyright (c) 1995, 1996, 1997, 1998, 2000
.\" 	Justin T. Gibbs.  All rights reserved.
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.Dd September 29, 2025
.Dt AHC 4
.Os
.Sh NAME
.Nm ahc
.Nd Adaptec VL/ISA/PCI SCSI host adapter driver
.Sh SYNOPSIS
To compile this driver into the kernel,
place the following lines in your
kernel configuration file:
.Bd -ragged -offset indent
.Cd "device scbus"
.Cd "device ahc"
.Pp
For one or more PCI cards:
.Cd "device pci"
.Ed
.Pp
Alternatively, to load the driver as a
module at boot time, place the following lines in
.Xr loader.conf 5 :
.Bd -literal -offset indent
ahc_load="YES"
ahc_isa_load="YES"
ahc_pci_load="YES"
.Ed
.Sh DESCRIPTION
This driver provides access to the
.Tn SCSI
bus(es) connected to the Adaptec AIC77xx and AIC78xx
host adapter chips.
.Pp
Driver features include support for twin and wide busses,
fast, ultra or ultra2 synchronous transfers depending on controller type,
tagged queueing, SCB paging, and target mode.
.Pp
Per target configuration performed in the
.Tn SCSI-Select
menu, accessible at boot
is honored by this driver.
This includes synchronous/asynchronous transfers,
maximum synchronous negotiation rate,
wide transfers,
disconnection,
the host adapter's SCSI ID.
For systems that store non-volatile settings in a system specific manner
rather than a serial eeprom directly connected to the aic7xxx controller,
the
.Tn BIOS
must be enabled for the driver to access this information.
This restriction applies to
many chip-down motherboard configurations.
.Pp
Performance and feature sets vary throughout the aic7xxx product line.
The following table provides a comparison of the different chips supported
by the
.Nm
driver.
Note that wide and twin channel features, although always supported
by a particular chip, may be disabled in a particular motherboard or card
design.
.Bd -ragged -offset indent
.Bl -column "aic7895CX" "MIPSX" "PCI/64X" "MaxSyncX" "MaxWidthX" "SCBsX" "2 3 4 5 6 7 8X"
.It Em "Chip" Ta "MIPS" Ta "Bus" Ta "MaxSync" Ta "MaxWidth" Ta "SCBs" Ta "Features"
.It "aic7770" Ta "10" Ta "VL" Ta "10MHz" Ta "16Bit" Ta "4" Ta "1"
.It "aic7850" Ta "10" Ta "PCI/32" Ta "10MHz" Ta "8Bit" Ta "3" Ta ""
.It "aic7860" Ta "10" Ta "PCI/32" Ta "20MHz" Ta "8Bit" Ta "3" Ta ""
.It "aic7870" Ta "10" Ta "PCI/32" Ta "10MHz" Ta "16Bit" Ta "16" Ta ""
.It "aic7880" Ta "10" Ta "PCI/32" Ta "20MHz" Ta "16Bit" Ta "16" Ta ""
.It "aic7890" Ta "20" Ta "PCI/32" Ta "40MHz" Ta "16Bit" Ta "16" Ta "3 4 5 6 7 8"
.It "aic7891" Ta "20" Ta "PCI/64" Ta "40MHz" Ta "16Bit" Ta "16" Ta "3 4 5 6 7 8"
.It "aic7892" Ta "20" Ta "PCI/64" Ta "80MHz" Ta "16Bit" Ta "16" Ta "3 4 5 6 7 8"
.It "aic7895" Ta "15" Ta "PCI/32" Ta "20MHz" Ta "16Bit" Ta "16" Ta "2 3 4 5"
.It "aic7895C" Ta "15" Ta "PCI/32" Ta "20MHz" Ta "16Bit" Ta "16" Ta "2 3 4 5 8"
.It "aic7896" Ta "20" Ta "PCI/32" Ta "40MHz" Ta "16Bit" Ta "16" Ta "2 3 4 5 6 7 8"
.It "aic7897" Ta "20" Ta "PCI/64" Ta "40MHz" Ta "16Bit" Ta "16" Ta "2 3 4 5 6 7 8"
.It "aic7899" Ta "20" Ta "PCI/64" Ta "80MHz" Ta "16Bit" Ta "16" Ta "2 3 4 5 6 7 8"
.El
.Pp
.Bl -enum -compact
.It
Multiplexed Twin Channel Device - One controller servicing two busses.
.It
Multi-function Twin Channel Device - Two controllers on one chip.
.It
Command Channel Secondary DMA Engine - Allows scatter gather list and
SCB prefetch.
.It
64 Byte SCB Support - SCSI CDB is embedded in the SCB to eliminate an extra DMA.
.It
Block Move Instruction Support - Doubles the speed of certain sequencer
operations.
.It
.Sq Bayonet
style Scatter Gather Engine - Improves S/G prefetch performance.
.It
Queuing Registers - Allows queueing of new transactions without pausing the
sequencer.
.It
Multiple Target IDs - Allows the controller to respond to selection as a
target on multiple SCSI IDs.
.El
.Ed
.Sh CONFIGURATION OPTIONS
.Pp
To allow PCI adapters to use memory mapped I/O if enabled:
.Pp
.Cd options AHC_ALLOW_MEMIO=(0 -- disabled, 1 -- enabled)
.Bd -ragged -offset indent
Memory mapped I/O is more efficient than the alternative, programmed I/O.
Most PCI BIOSes will map devices so that either technique for communicating
with the card is available.  In some cases, usually when the PCI device is
sitting behind a PCI->PCI bridge, the BIOS may fail to properly initialize
the chip for memory mapped I/O. The typical symptom of this problem is a
system hang if memory mapped I/O is attempted.
.Pp
Most modern motherboards perform the initialization correctly and work fine
with this option enabled and it is the default. This option can also be
dynamically configued through a device hint documented below.
.Ed
.Pp
To statically configure one or more controllers to assume the target role:
.Pp
.Cd options AHC_TMODE_ENABLE=<bitmask of units>
.Bd -ragged -offset indent
The value assigned to this option should be a bitmap of all units where target
mode is desired. For example, a value of 0x25, would enable target mode on
units 0, 2, and 5. A value of 0x8a enables it for units 1, 3, and 7.
.Pp
Note that controllers can be dynamically configured through a device hint
documented below.
.Ed
.Sh BOOT OPTIONS
The following options are switchable by setting values in
.Pa /boot/device.hints .
.Pp
They are:
.Bl -tag -width indent
.It Va hint.ahc. Ns Ar N Ns Va .tmode_enable
A hint to define whether the SCSI target mode is enabled, defaults to disabled
(0 -- disabled, 1 -- enabled).
.It Va hint.ahc. Ns Ar N Ns Va .allow_memio
A hint to define whether memory mapped io is enabled or disabled for this
adapter, defaults to enabled (0 -- disabled, 1 -- enabled).
.El
.Sh HARDWARE
The
.Nm
driver supports the following VL/ISA/PCI parallel SCSI controllers and cards:
.Pp
.Bl -bullet -compact
.It
Adaptec
.Tn AIC7770
host adapter chip
.It
Adaptec
.Tn AIC7850
host adapter chip
.It
Adaptec
.Tn AIC7860
host adapter chip
.It
Adaptec
.Tn AIC7870
host adapter chip
.It
Adaptec
.Tn AIC7880
host adapter chip
.It
Adaptec
.Tn AIC7890
host adapter chip
.It
Adaptec
.Tn AIC7891
host adapter chip
.It
Adaptec
.Tn AIC7892
host adapter chip
.It
Adaptec
.Tn AIC7895
host adapter chip
.It
Adaptec
.Tn AIC7896
host adapter chip
.It
Adaptec
.Tn AIC7897
host adapter chip
.It
Adaptec
.Tn AIC7899
host adapter chip
.It
Adaptec
.Tn 274X(W)
.It
Adaptec
.Tn 274X(T)
.It
Adaptec
.Tn 2910
.It
Adaptec
.Tn 2915
.It
Adaptec
.Tn 2920C
.It
Adaptec
.Tn 2930C
.It
Adaptec
.Tn 2930U2
.It
Adaptec
.Tn 2940
.It
Adaptec
.Tn 2940J
.It
Adaptec
.Tn 2940N
.It
Adaptec
.Tn 2940U
.It
Adaptec
.Tn 2940AU
.It
Adaptec
.Tn 2940UW
.It
Adaptec
.Tn 2940UW Dual
.It
Adaptec
.Tn 2940UW Pro
.It
Adaptec
.Tn 2940U2W
.It
Adaptec
.Tn 2940U2B
.It
Adaptec
.Tn 2950U2W
.It
Adaptec
.Tn 2950U2B
.It
Adaptec
.Tn 19160B
.It
Adaptec
.Tn 29160B
.It
Adaptec
.Tn 29160N
.It
Adaptec
.Tn 3940
.It
Adaptec
.Tn 3940U
.It
Adaptec
.Tn 3940AU
.It
Adaptec
.Tn 3940UW
.It
Adaptec
.Tn 3940AUW
.It
Adaptec
.Tn 3940U2W
.It
Adaptec
.Tn 3950U2
.It
Adaptec
.Tn 3960
.It
Adaptec
.Tn 39160
.It
Adaptec
.Tn 3985
.It
Adaptec
.Tn 4944UW
.It
Many motherboards with on-board
.Tn SCSI
support
.El
.Sh SCSI CONTROL BLOCKS (SCBs)
Every transaction sent to a device on the SCSI bus is assigned a
.Sq SCSI Control Block
(SCB).
The SCB contains all of the information required by the
controller to process a transaction.
The chip feature table lists
the number of SCBs that can be stored in on-chip memory.
All chips
with model numbers greater than or equal to 7870 allow for the on chip
SCB space to be augmented with external SRAM up to a maximum of 255 SCBs.
Very few Adaptec controller configurations have external SRAM.
.Pp
If external SRAM is not available, SCBs are a limited resource.
Using the SCBs in a straight forward manner would only allow the driver to
handle as many concurrent transactions as there are physical SCBs.
To fully utilize the SCSI bus and the devices on it,
requires much more concurrency.
The solution to this problem is
.Em SCB Paging ,
a concept similar to memory paging.
SCB paging takes advantage of
the fact that devices usually disconnect from the SCSI bus for long
periods of time without talking to the controller.
The SCBs for disconnected transactions are only of use to the controller
when the transfer is resumed.
When the host queues another transaction
for the controller to execute, the controller firmware will use a
free SCB if one is available.
Otherwise, the state of the most recently
disconnected (and therefore most likely to stay disconnected) SCB is
saved, via dma, to host memory, and the local SCB reused to start
the new transaction.
This allows the controller to queue up to
255 transactions regardless of the amount of SCB space.
Since the
local SCB space serves as a cache for disconnected transactions, the
more SCB space available, the less host bus traffic consumed saving
and restoring SCB data.
.Sh SEE ALSO
.Xr ahd 4 ,
.Xr cd 4 ,
.Xr da 4 ,
.Xr sa 4 ,
.Xr scsi 4
.Sh HISTORY
The
.Nm
driver appeared in
.Fx 2.0 .
.Sh AUTHORS
The
.Nm
driver, the
.Tn AIC7xxx
sequencer-code assembler,
and the firmware running on the aic7xxx chips was written by
.An Justin T. Gibbs .
.Sh BUGS
Some Quantum drives (at least the Empire 2100 and 1080s) will not run on an
.Tn AIC7870
Rev B in synchronous mode at 10MHz.
Controllers with this problem have a
42 MHz clock crystal on them and run slightly above 10MHz.
This confuses the drive and hangs the bus.
Setting a maximum synchronous negotiation rate of 8MHz in the
.Tn SCSI-Select
utility will allow normal operation.
.Pp
Although the Ultra2 and Ultra160 products have sufficient instruction
ram space to support both the initiator and target roles concurrently,
this configuration is disabled in favor of allowing the target role
to respond on multiple target ids.
A method for configuring dual role mode should be provided.
.Pp
Tagged Queuing is not supported in target mode.
.Pp
Reselection in target mode fails to function correctly on all high
voltage differential boards as shipped by Adaptec.
Information on
how to modify HVD board to work correctly in target mode is available
from Adaptec.
